1. Field of the Invention
The invention relates to the field of synchronizing signal separating circuits for television receivers.
2. Description of Related Art
In a television receiver, the line deflection circuit is provided with a high time constant in order to compensate for disturbances in the form of missing or temporarily wrongly phased line synchronizing pulses. A high time constant of this type is possible because the line synchronizing pulses coming from the transmitter are practically constant, with a high degree of accuracy in phase and frequency, and phase displacements in the line raster practically never occur.
In practice however, video signals having appreciable variations is in frequency and in the phasing of the line raster are also supplied to the television receiver. This is particularly the case when the video signal is delivered by a video recorder because the line frequency and the phasing of the line raster are then no longer constant, due to fluctuations in speed, stretching of the tape and other such effects. In particular, sudden alterations in the phasing of the line raster occur when the scanning video head runs at an angle across a plurality of adjacent tracks during a search process in a video recorder. If the time constant of the line deflection means in the television receiver then has the high value, the line deflection means cannot follow the new phase quickly enough and there will be geometrical distortions in the reproduced image. It is then necessary, and known, to switch the time constant in the line synchronizing means to a lower value for such a signal.
The sought after high degree of immunity to disturbance thus leads to a high time constant, while a signal having line pulses fluctuating in frequency or phase requires a smaller time constant. Consequently, a compromise is required as regards the dimensioning of the line deflection circuit in order to satisfactorily meet the two demands.
Known separating circuits can contain an amplitude selective stage and a flywheel stage which serves for suppressing disturbances. An adequate suppression of disturbance requires a flywheel stage which introduces a relatively high time constant in order to exclude the influence of missing individual synchronizing pulses or synchronizing pulses which are wrongly located in time. However, a rapid adaptation to alterations in frequency and phase of the line synchronizing pulses, for a signal from a video recorder for example, requires a small time constant. In practice, a compromise has to be struck between these two contrasting requirements or the time constants are switched in dependence on the type of signal.